Test and application of a non-destructive photo-method investigating the parasitic stage of the threatened mussel Margaritifera margaritifera on its host fish E. Salmo trutta

► A non-destructive photo-method investigated encystment of a unionid mussel on its host fish. ► The method reliably estimated encystment without injuring mussels or host fish. ► When applied to individual streams, mean encystment intensity was estimated for YOY and older fish. ► The photo-method may be used for other unionid mussel species. ► It may now be possible to investigate every life stage of unionids without using harmful methods.